As illustrated in FIG. 6, a welding start circuit ST is arranged outside of a welding power source PS. When receiving a welding start signal St from the welding start circuit ST, the welding power source PS outputs a welding voltage Vw and a welding current Iw to generate an arc and a feed control signal Fc to control the feed of a welding wire 1. As the welding start circuit ST, a programmable logic controller (PLC), which controls welding steps, and a robot controller may be employed. A feed roll 5 is connected to a wire feed motor WM. The welding wire 1 is fed to a base material 2 via the interior of a welding torch 4 through rotation of the feed roll 5. An arc 3 is generated between the welding wire 1 and the base material 2 in a shield gas 6 by feeding power to the welding wire 1 through a feeder chip. When the wire feed motor WM rotates in a forward direction, the welding wire 1 is sent in a direction toward the base material 2 and thus proceeds. In contrast, when the wire feed motor EM rotates in a reverse direction, the welding wire 1 is moved in a direction separating from the base material 2 and thus retracted.
When the welding wire 1 and the base material 2 are held in contact (short-circuited) or an arc is generated, the welding current Iw flows between the welding wire 1 and the base material 2. Contrastingly, when the welding wire 1 and the base material 2 are separate from each other and the current state is a no-load state in which the arc 3 is not generated, the welding voltage Vw becomes a maximum value (a no-load voltage) and the welding current Iw does not flow between the welding wire 1 and the base material 2. The distance between the distal end of the welding wire 1 and the base material 2 is a wire distal end/base material distance Lw [mm]. Accordingly, the wire distal end/base material distance Lw is substantially equal to the arc length when an arc is generated.
FIG. 7 includes timing charts representing a conventional retract arc start control method performed by the welding apparatus illustrated in FIG. 6. FIG. 7(A) represents the welding start signal St, and FIG. 7(B) represents the feed control signal Fc. FIG. 7(C) represents the welding voltage Vw, and FIG. 7(D) represents the welding current Iw. FIG. 7(E) represents the wire distal end/base material distance Lw. The retract arc start control method will now be described with reference to FIGS. 7(A) to 7(E).
(1) Wire Slowdown Period from Time Point t1 to Time Point t2
At time point t1, with reference to FIG. 7(A), the welding start signal St is input and reaches a high level. Then, as represented by FIG. 7(B), the feed control signal Fc becomes a slow-down feed speed Fir and the welding wire starts to proceed. Normally, the slow-down feed speed Fir is set to a slow speed of approximately 1 to 2 m/min. This is because if the slow-down feed speed is raised, the arc start performance is deteriorated. Simultaneously, output of the welding power source PS is started and, with reference to FIG. 7(C), the welding voltage Vw is applied. Since the state at time point t1 is the non-load state, the welding voltage Vw is set to a non-load voltage Vn1. After time point t1, the welding wire proceeds and the wire distal end/base material distance Lw gradually decreases as represented by FIG. 7(E).
(2) Short Circuit Period from Time Point t2 to Time Point t3
When the distal end of the wire contacts the base material at time point t2, the wire distal end/base material distance Lw becomes zero as represented by FIG. 7(E), and the welding voltage Vw becomes a short circuit voltage of approximately several volts as represented by FIG. 7(C). Further, the welding current Iw becomes an initial current setting value Iir with reference to FIG. 7(D). The initial current setting value Iir is a low current value of approximately 10 A to 100 A. At this stage, by detecting the fact that the welding voltage Vw has become smaller than or equal to a reference voltage Vth as represented by FIG. 7(C), it is determined that the welding wire has been brought into contact with the base material. Further, at this stage, with reference to FIG. 7(B), the feed control signal Fc becomes a retract feed speed setting value Fbr having a negative value, and thus retract of the welding wire is started. However, in the short circuit period from time point t2 to time point t3, as represented by FIG. 7(E), the distal end of the wire and the base material are maintained in contact with each other due to a delay time caused by reversal of the rotation of the wire feed motor from the forward direction to the reverse direction or a delay time necessary for retracting the welding wire by a length corresponding to the play of the welding wire in the welding torch. Although the short circuit period varies depending on the type of the wire feed motor and the length of the welding torch, the short circuit period is normally 10 to 100 ms.
(3) Initial Arc Lift Period Ti from Time Point t3 to Time Point t4
When the distal end of the wire is separated from the base material as represented by FIG. 7(E), a current corresponding to the initial current setting value Iir is supplied and an initial arc is generated. When the initial arc is produced, with reference to FIG. 7(C), the welding voltage Vw reaches an arc voltage of several tens of volts, which exceeds the reference voltage Vth. In the predetermined initial arc lift period Ti (from time point t3 to time point t4), the welding wire is retracted continuously as represented by FIG. 7(B). This is because, if movement of the welding wire is switched from retract to proceed immediately after the initial arc has been produced, the wire and the base material may be caused to re-contact with each other due to an insufficient arc length. In order to prevent such re-contact and smoothly switch to a steady arc state, the welding wire is continuously retracted to increase the arc length with the initial arc maintained in the initial arc lift period. Retract of the welding wire is continued until the arc length becomes substantially equal to a steady arc length. The initial current for the initial arc is maintained at the low level in order to prevent the initial arc from melting the distal end of the wire and causing the arc to flare up. If the arc flares up when the welding wire is retracted, it is difficult to raise the arc accurately to a desirable value.
(4) Steady Arc State Period After Time Point t4
When the initial arc lift period Ti ends at time point t4, the feed control signal Fc becomes a steady feed speed setting value Fcr as represented by FIG. 7(B) and the welding wire re-starts to proceed. Simultaneously, with reference to FIG. 7(C), the welding voltage Vw is controlled to become equal to a predetermined voltage set value Vr, and, as represented by FIG. 7(E), a steady welding current Ic corresponding to the steady feed speed is supplied. In this manner, with reference to FIG. 7(E), the initial arc generating state is smoothly switched to the steady arc state. In the steady arc state, the arc represents a steady arc length Lc.
In the above-described control method, constant current control is performed on the initial current by the welding power source PS so as to control the current accurately. As represented by FIG. 7(D), the initial current is constant. However, there may be cases in which the current is suppressed to a small value when the welding wire contacts the base material at time point t2 and then increased in the short circuit period. This prevents an arc from being generated, and melting and joining the welding wire and the base material together when the welding wire and the base material are in contact. The above-described conventional art is disclosed in, for example, Patent Documents 1, 2.
In the conventional art illustrated in FIG. 7, the initial current, which has a lower current value than the current of the steady state, is supplied in the initial arc lift period Ti from time point t3 to time point t4. Accordingly, the heat provided to the base material becomes insufficient and, by time point t4, at which the welding wire re-starts to proceed at the steady feed speed and feed of the steady welding current Ic is started, a sufficient weld pool has not yet been formed in the base material. As a result, when the welding wire melts and produces droplets after time point t4, the droplets cannot completely transfer to the base material, thus causing spatter. Since the amount of the spatter is small, the spatter does not cause any problem in normal arc start methods other than the retract arc start method. However, since the retract arc start method is employed in high-quality welding, even a small amount of spatter may cause a problem.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2006-231414
Patent Document 2: Japanese Laid-Open Patent Publication No. 2007-30018